Process of obtaining thiodiglycolic acid



Aug. 5, 1947.

A.` E. BEARSE rnocnss or OBTAINING TaIoDIGLYcoLIc AcI-D Filed H'ay 18, 1945 #IES llll I-.

n 2o.... .BCG l- .Patented Aug. 5, 1947 2,425,224. PROCESS F OBTAANCIIBIIG THIODIGLYCOLIC Arthur E: Bearse, Columbus, Ohio, assigner, by mesne assignments, to The C. P. Hall Company, Akron, Ohio, a corporation of Ohio i Application May 18, 1945, Serial No. 594,582

9 claims. (ci. 26o-481)' This invention relates to an improved method of obtaining thiodiglycolic acid and water-soluble 4alkyl-substituted thiodiglycolic acids. In particular, it relates to a process for -recovering the above-acids from aqueous inorganic salt solutions resulting from their production. It includes the conversion of the acids to'esters by a process which is a continuation of the recovery process.

Thiodiglycolic acid and its alkyl-substitution products have been prepared ordinarily by the reaction of a salt of an alpha-halogenated fatty acid with an alkali metal sulfide in aqueous solution, followed by acidification with a mineral acid, such as sulfuric or hydrochloric, to liberate the thiodiglycolic acid. The following equations'illustrate the reaction:

R1 and R2 in the above formulae represent hydrogen atoms or lower alkyl groups which form water-soluble acids, and ordinarily alkyl groups which contain no more than about three carbon atoms. Such acids include, for example, the following:

HOOC.C (CH3) 2.S.C (CH3) IrCOOH and HOOC.HC(C2H5) .S.HC(C2H5)'.COOH

Because of the fact that thiodiglycolic acid and the lower alkyl-substituted thiodiglycolic acids are very soluble in water, isolation of the pure material from the mixture of acid, inorganic salts, and wate;` presents seriousdifculties.

According to the prior art, the aqueous solution of thiodiglycolic acid resulting from acidication of the aqueous sodium thiodiglycolate-is subjected to crystallization, whereupon only part of the thiodiglycolic acid separates from solution. A considerable portion remains dissolved in the motherl liquors together with inorganic salts. Moreover, the portion of thiodiglycolic acid which crystallizes out is .contaminated with inorganic salts and must be further purified to obtain a satisfactory product.

- rounded'the preparation of thiodiglycolic acid in the past, this acid has not been used industrially in spite of the fact that it can be made from inexpensive -raw materials such as sodium sulfide,

acetic acid, and chlorine.

Thiodiglycolic. acid is a useful dibasic acid,. which mayv replace other well-known dibasic acids,.such as adipi-c, azelaic, sebacic, maleic, and phthalic acids, in certainv applications. Certain esters obtainedfrom .thiodiglycolic acid and aliphatic and cycloaliphatic alcohols have been found useful as plasticizers for synthetic rubber and certain types ofv resins, Esters of the alkylsubstitutedpacids also mayy be useful as .plasticizers.-

It is anf'object of thisk invention to provide a simple, economical, and convenient method for recovering thiodi'glycolic acid and water-soluble alkyl-substituted thiodiglycolic acids in substantially pure form from aqueous inorganic salt solutions obtained in the manufacture of these acids from alkali metal suldes and alpha-halogenated fatty acid salts.

Other objects and advantages of this invention will become apparent from the following detailed description:

In accordance with this invention it has been found that the difliculties mentioned above can be overcome and. that thiodiglycolic acid and water-soluble, alkyl-substituted thiodiglycolic acids can be recovered readily in good yield from aqueous inorganic salt-containing solutions in which they are produced by extracting said solutions with an aliphatic or. cycloaliphatic ketone which contains 4 to' 6 carbon atoms. Such ketones form water-azeotropes, they are not completely miscible with the aqueous solution of the acid and on subsequent distillation leave an aqueous solution from which the acid may be recovered readily. The solvent properties of such compounds are easily altered by the addition of a small amount of benzene or carbon tetrachloride, for example. The ketones containing less than four carbon atoms are, of themselves, too soluble in aqueous inorganic salt solutions to be suitable for the extraction. Ketones containing more than six carbon atoms do not dissolve enough of the acids to make them effective as extractants.-

Extraction of the thiodiglycolic acid may be carried out batchwise or by a continuous coun- `order to increase the solubility of the thiodiglycolic acid in the organic solvent, but with my 'vents listed above may be used in conjunction with other organic solvents for carrying out the extraction process of this invention.

While this specication describes in detail the process of producing thiodiglycolic acid it will be obvious to one skilled in the art that the procedure may be applied also to the preparation of alkyl-substituted thiodiglycolic acids of the general formula:

where R1 and Rz represent hydrogen atoms or alkyl groups in water-soluble, alkyl-substituted acids. Acids of the above type are formed in a well-known manner by reacting an alpha-chloro or alpha-bromo fatty acid salt wit-h an inorganic sulfide, such as sodium or potassium sulfide, in aqueous solution, followed by acidification with a mineral acid, such as sulfuric or hydrochloric acid. Representative examples of alpha-chloro and alpha-bromo fatty acids are alpha-chloroacetic acid, alpha-chloropropionic acid, alpha- Abromopropionic acid, alpha-chlorobutyric acid, and alpha-chloroisobutyric acid. Ordinarily, the sodium salt of the acid is used because most economical. Other salts might be employed.

In extracting thiodiglycolic acid from the crude aqueous solution containing inorganic salts, I prefer to use a continuous countercurrent process because of the simplicity of operation. For example, a suitable solvent is passed upward through an extraction tower, provided with suitable packing or other arrangement for obtaining intimate contact between the solvent and the aqueous solution, and the aqueous solution containing inorganic salts and thiodiglycolic acid is passed downward through the tower..

From the top of the extraction tower there is withdrawn a liquid stream consisting of a solution of thiodiglycolic acid in the organic solvent. The thiodiglycolic acid can be crystallized out in substantially pure form after distilling oif-part of the solvent. The solvent may be recovered and recycled to the extraction tower. Part of the water which is dissolved in the organic extract solution will be distilled azeotroplcally with the solvent; with certain solvents al1 of the water may be removed in this way.

From the bottom of the extraction tower there is withdrawn an aqueous solution of inorganic salts containing very little thiodiglycolic acid. Small quantities of organic solvent contained in this aqueous solution may be stripped o by distillation and recovered.

In order to avoid crystallization of inorganic salts in the extraction tower due to removal of water by the organic solvent, it is sometimes deagitator I3.

slrable to introduce a small amount of water along with the organic solvent. i

The accompanying drawing shows diagrammatically one method of carrying out the process of this invention as applied to the manufacture of thiodiglycolic acid. An' aqueous solution of sodium sulfide is introduced through lines I0 and Il into a jacketed reactor I2, provided with an An aqueous solution of sodium chloroacetate, prepared by neutralizing chloroacetic acid with sodium carbonate or sodium hydroxide, is introduced through line'Il. The mixture is heated until reaction is substantially complete, and then sulfuric acid is added through lines I5 and II in sulcient quantity to convert the sodium thiodiglycolate to thiodiglycolic acid.

The aqueous solution of thiodiglycolic acid and inorganic salts is withdrawn from reactor I2 through valved line I6 and passes through filter I'I to extraction tower I8 which may be provided with suitable packing to insure elcient contact between the aqueous phase and the methyl ethyl ketone which is introduced near the,bottom of tower I8 through line I9. The methyl ethyl ketone ascends the tower and removes the thiodiglycolic acid from the descending aqueous layer.

From the top of extraction tower I8 there is withdrawn through valved line 20 a solution of thiodiglycolic acid in methyl ethyl ketone. This solution passes to evaporator 2l where most of the solvent, together with the small amount of water that may be present, is distilled off. This evaporator may be operated under vacuum if desired. The vapors leaving the evaporator through lines 22 and 23 are Passed to condenser 24, and the condensate is conveyed to the methyl ethyl ketone storage tank 25, from which it may be withdrawn and returned to the extraction tower through line I9. Methyl ethyl ketone required for make-up may be added through line 26.

A concentrated solution of thiodiglycolic acid in methyl ethyl ketone is withdrawn from evaporator 2| through line 21 and conducted to crystallizer 28. The mother liquor from the crystallizer is returned to evaporator 2l through line 29. The crystalline thiodiglycolic acid from crystallizer 28 is conveyed to drier 30 and thence to storage. Residual solvent which is removed from the crystals in the drier is conveyed through line 23, thence to condenser 24 and storage tank 25.

The aqueous salt solution from which the thiodiglycolic acid has been extracted leaves extraction tower I8 through line 3l and is conducted to a stripping column 32 for recovery of the methyl ethyl ketone which it contains. Steam is introduced through line 33 and the azeotrope of methyl ethyl ketone leaving the stripping column through line 34 passes through condenser 35 to storage tank 25. Waste solution of inorganic salts is discarded from the system through line 36.

It is obvious that other methods may be used for recovering crystalline thiodiglycolic acid from solution in the methyl ethyl ketone or other ketone extractant. For example, an evaporator drier consisting of enclosed steam heated rolls may be employed; or the steps of evaporation, crystallization, and drying may be carried out in other conventional types of equipment.

A lmodification of the above process, not shown in-the diagram, is to remove the methyl ethyl ketone or other solvent from the extract by treating with water (as steam, if preferred) in excess of the quantity required to form the methyl ethyl aqueous solution.

ketone-water azeotrope, and fractionally distilling off the latter so as to leave a concentrated aqueous solution of thiodiglycollc acid. The thiodiglycolic acid can then be crystallized from this The -azeotrope referred to mately 2 kilograms of crystalline thiodiglycolic above may be condensed and reused for further extractions. One advantage of this modied method of recovering the thiod'iglycolic acid Ais that it eliminates losses of methyl ethyl ketone which may be associated with the crystallization and drying steps.

. Still another modification of the process may be employed if it is desired to use the thiodiglycolic acid in the ketone solution for the preparation of an ester. By this modification it is not necessary to isolate the crystalline thiodiglycolic acid; but, instead, the alcohol from which the ester is to .be made is added to the ketone solution of thiodiglycolic acid, and the lower boiling ketone is fractionated ofi and recovered. The remaining solution of thiodiglycolic acid in the alcohol is then reacted in the conventional manner to produce an ester of thiodiglycolic acid with said alcohol, of thiodiglycolic acid in methyl ethyl ketone there is added 2-ethylhexyl alcohol slightly in excessv of the amount required to esterify the acid present. The methyl ethyl ketone is removed by distillation, sulfuric acid is added to the residual solution as a catalyst, and the mixture is refluxed with azeotropic removal of water until esterification is substantially complete. Di-2-ethylhexyl thiodiglycolate is thus formed.

The following examples further illustrate the process of this invention, Example 1 describes the preparation of thiodiglycolic acid in aqueous solution followed by batch extraction with methyl ethyl ketone. Example 2 describes a process of |countercurrently extracting the acid. Example 3 describes a process of preparing an ester from a ketone extract of the acid.

Example 1 boil and refluxed for approximately two hours to complete the reaction. The reaction mixture was cooled to 30 C., and 2.2 kilograms of concentrated sulfuric acid were added over a period of about ten minutes. During the entire process described above, the mixture was mechanically agitated.

In order to recover the thiodiglycolic acid, the acidied reaction mixture was extracted with methyl ethyl ketone in the following manner: Three kilograms of methyl ethyl ketone were acid. Additional thiodiglycolic acid to the extent of 600 grams was recovered by working up the mother liquors. The total over-all yield of thiodiglycolic acid was 83 per cent of the theoretical. The product was substantially free from inorganic salts.

Instead of isolating the thiodiglycolic acid as described in the preceding paragraph, the following procedure was also found to be satisfactory: The combined extracts were diluted with approximately 5 kilograms of water and subjected to For example, to a solution Raschig rings.

added to the kettle, and the mixture was agitated l for five minutes and allowed to separate into two layers. The upper layer was withdrawn, and the procedure was repeated four times with fresh methyl ethyl ketone.

' 'I'he combined extracts were concentrated to a volume of approximately five liters. The concentrated solution on cooling deposited approxiv fractional distillation, whereupon the methyl ethyl ketone distilled over azeotropically with water at about 73 C. The thiodiglycolic acid, which remained in the aqueous solution in the still pot after removalof the ketone, was recovered by further evaporation and crystallization.

Example 2 tracted countercurrently in the following manner:

The extraction column used for this process was 28 inches in length and 3 inches in diameter. It was packed with 1pinch porcelain another inlet above the packing for introducing the aqueous solution. An outlet was provided at the top of the column for withdrawal of the ketone solution, and another outlet was provided at the bottom for withdrawal of the spent aqueous solution. The tower was operated in such a manner that the methyl ethyl ketone formed the continuous phase, and the aqueous solution formed the dispersed phase.

Methyl ethyl ketone containing about 20 per cent water and the aqueous inorganic-salt-containing solution of thiodiglycolic acid were pumped into the tower through the appropriate inlets, each at vthe rate of approximately 22 liters per hour. The methyl ethyl ketone solution Withdrawn from the top of the tower was concentrated and crystallized as described in Example 1. The extraction efliciency was 93 per cent, and the over-all yield of thiodiglycolic acid was 8l per cent of the theoretical. The thiodiglycolic acid thus obtained was substantially devoid of inorganic salts,

Eample 3 An ester may be produced from the methyl ethyl ketone solution of thiodiclycolic acid obtained as described inl Example 1, by proceeding is discarded. After about two and one-half hours the esterication is substantially complete.

'I'he crude esterification mixture is washed` with water, then with sodium carbonate solution, andagain with water until neutral. The benzene is distilled olf, using vacuum for completion of the removal of the benzene. After removal of An inlet was provided below the, packing for introducing methyl ethyl ketone and The benzene layer is rel turned to the esteriiication reactor, and the water excess Z-ethylhexanol, di-2-ethylhexyl thiodiglycolate boiling at 225235 C. at a pressureof 5 to 6 mm. of mercury is obtained in good yield.

Other ketone extractants may be similarly employed andthe acids obtained as such, or the solutions may be utilized in various ways as the above illustrates.

What I claim is:

1. The method of obtaining an acid of the class consisting of thiodiglycolic acid and water-soluble, alkyl-substituted thiodiglycolic acids from the aqueous inorganic salt solution in which it is produced by reacting an alkali metal sulde with an alpha-halogenated fatty acid salt in Water solution and then acidifying with a mineral acid, which comprises extracting the acid from the aqueous solution with a ketone from the class consisting of ketones containing four to six carbon atoms.

2. The method of obtaining an acid of the class consisting of thiodiglycolic acid and water-soluble, alkyl-substituted thiodiglycolic acids from the aqueous inorganic salt solution in which it is produced by reacting an alkali metal sulfide with an alpha-halogenated fatty acid salt in water solution and then acidifying with a mineral acid, which comprises extracting the acid from the aqueous solution with a ketone from the class consisting of the ketones which contain 4 to 6 carbon atoms, adding water to the extract in ex-- cess of that required to form the ketone-water azeotrope, and distillng off the latter to leave an aqueous solution of the acid.

3. The method of obtaining thiodiglycolic acid from the aqueous inorganic salt solution in which it is produced by reacting an alkali meta] sulfide with an alpha-halogenated acetic acid salt and then acidifying with a mineral acid, which comprises extracting the thiodiglycolic acid from the aqueous solution with methyl ethyl ketone.

4. The method of obtaining thiodiglycolic acid from the aqueous inorganic salt solution in which it is produced by reacting an alkali metal sulde with an alpha-halogenated acetic acid salt and then acidifying with a mineral acid, which comprises extracting the thiodiglycolic acid from the aqueous solution with methyl ethyl ketone, adding water to the extract in excess of that required to form the ketone-water azeotrope, and distillng off the latter to leave an aqueous solution of the acid.

5. The continuous process of producing an acid of the class consisting of thiodiglycolic acid and the lower alkyl-substituted derivative acids which are relatively soluble in water, which comprises reacting an alkali metal sulde with an alphahalogenated fatty acid in water solution and then acidifying with a mineral acid to obtain the acid of the aforesaid class, extracting the acid by bringing a substantially constantly flowing stream of it into large-surface contact with a substantially constant countercurrently owing stream of a ketone of the class consisting of ketones which contain 4 to 6 carbon atoms, separating the immiscible extractant by gravity and lrecovering the acid therefrom, all as steps of a substantially continuous process.

6. The continuous process of producing an acid of the class consisting of thiodiglycolic acid and the lower alkyl-substituted derivatives which are relatively water soluble, which comprises reacting an alkali metal sulde with an alpha-halogenated fatty acid in water solution and then acidifying with a mineral acid to obtain the acid of the aforesaid class, extracting the acid .by bringing a substantially constantly owing stream of it into large-surface contact with a substantially constant countercurrently flowing stream of a ketone of the class consisting of ketones which contain 4 to 6 atoms, separating the imrniscible extractant by gravity, adding water thereto to bring the water content of the mixture in excess of that required to form the water azeotrope with the solvent, and then distilling the water azeotrope of the solvent to leave an aqueous solution of the acid, all as steps of a substantially continuous process.

7. 'I'he continuous process of producing thiodiglycolic acid which comprises reacting an alkali metal sulfide with an alpha-halogenated acetic acid in water solution and then acldifying with a mineral acid to obtain the thiodiglycolic acid, extracting the acid by bringing thewater solution and methyl ethyl ketone into intimate, large-surface contact While causing the aqueous solution and ketone to flow countercurrent to one another, separating the ketone and aqueous solution by gravity, adding water to the resultant acid-containing ketone to make its water content in excess of that required to form a water-azeotrope with the ketone, distillng from the solution the water azeotrope of the ketone, and thus obltining as a residue a water solution of the aci 8. 'I'he method of obtaining an ester of the class consisting of esters of thiodiglycolic acid and water-soluble alkyl-substituted thiodiglycolic acids, which comprises extracting with a ketone of the class consisting of ketones containing four to six carbon atoms the aqueous inorganic salt solution in which the acid 4is produced by reacting an alkali metal sulfide -with the appropriate alpha-halogenated fatty acid in water solution and then acidifying with a mineral acid, adding to the ketone extract an alcohol of higher boiling point than the ketone and then removing the ketone from the resulting admixture by distillation and refluxing the resulting alcohol solution with an esteriiicaton catalyst to produce the ester of the acid.

9. The method of obtaining thiodiglycolic acid from the aqueous inorganic salt solution in which it is produced by reacting an alkali metal suliide with an alpha-halogenated acetic acid salt and then acidifying with a mineral acid, which comprises extracting the thiodiglycolic acid from the aqueous solution with a ketone from the class consisting of the ketones which contain 4 to 6 carbon atoms.

ARTHUR E'. BEARSE.

REFERENCES CITED The following references are of record in the le of this patent:

OTHER REFERENCES Schulze, Loven etc., Beilstein (4th ed. 1921), vol. III, p. 253. 

